Fix typescript types. (#23771)
Fix typescript types. (#23771)

Fix typescript type for cache and null check in mutation result.

## Bug

- fixes : https://github.com/vercel/next.js/issues/23770

**That issue deals with -**

- Return the codeflow if data is not returned from the mutation, and that fixes this warning.
![Screenshot from 2021-04-07 18-24-44](https://user-images.githubusercontent.com/31458531/113873069-18353a80-97d2-11eb-8c8c-9e9c9d280f95.png)

- Supply typescript typings for the target cache object.
![Screenshot from 2021-04-07 18-52-15](https://user-images.githubusercontent.com/31458531/113873504-80841c00-97d2-11eb-9b14-17d16a524c84.png)

Thanks.!
zeit/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js